This invention relates to a yarn withdrawal nozzle for open-end spinning arrangements which has an essentially funnel-shaped contact surface serving as the deflecting guide for a spun yarn.
In the case of open-end rotor spinning machines, for example, in the case of the rotor spinning machine sold by the firm W. Schlafhorst AG & Co., Monchengladbach, under the tradename of "Autocoro", yarn withdrawal nozzles are used which consist of a ceramic material on an aluminum oxide base. Such yarn withdrawal nozzles made of ceramics withstand the high wearing stress which occurs in the case of such open-end rotor spinning machines which currently are operated at rotational speeds of the spinning rotors of up to 130,000 min.sup.-1. However, when synthetic fibers, particularly polyester fibers or mixtures of natural fibers and synthetic fibers are processed, the operating speeds of such open-end rotor spinning machines are limited. The open-end rotor spinning machines cannot be operated at the possible high rotational speeds because damage by overheating caused by frictional heat occurs at the synthetic fibers. It is therefore customary to operate the open-end rotor spinning machines at a reduced speed, that is not at the maximally possible rotational rotor speed, when synthetic fibers are to be processed. In comparison to the theoretically possible rotational rotor speeds, this represents a production loss because the production of an open-end rotor spinning machine is directly proportional to the rotational rotor speed. This problem of damage to synthetic fibers or chemical fibers has been known for a long time and a satisfactory solution has not been found in this respect. It is known, for example, (German Patent Document DE 24 10 940 C3) to direct a cooling air flow onto a yarn guiding funnel which corresponds to a yarn withdrawal nozzle with respect to its function. However, this has not led to a useful solution.
It is an object of the invention to provide the prerequisites for enabling the open-end rotor spinning arrangements to run at an increased speed and particularly at increased rotational rotor speeds and, in the process, to be able to also spin synthetic fibers or chemical fibers without any damage to them.
This object is achieved according to preferred embodiments of the invention by providing that at least the contact surface of the yarn withdrawal nozzle consists of a material which has a thermal conductivity of at least 80 W/mK (Watts per meter Kelvin) in a temperature range of approximately 50.degree. C. to approximately 100.degree. C.
A damaging of the fibers can only be avoided if the generated frictional heat between the fibers and the contact surface of the yarn withdrawal nozzle does not result in an unacceptably high temperature of the fibers. The invention is therefore based on the consideration that the generating of the frictional heat itself, which is a function of the coefficient of friction, of the normal force between the yarn and the contact surface of the withdrawal nozzle and of the sliding speed, cannot be reduced significantly for reasons of spinning technology. It is therefore provided that the yarn withdrawal nozzle will be capable of dissipating the frictional heat sufficiently and also as fast as possible so that the undissipated portion of the frictional heat does not result in an exceeding of a critical temperature for the fibers and thus to a local overheating of these fibers. In this case, the invention is also based on the recognition that it is not the withdrawal speed of the yarn that is the main cause of the previous fiber damage but rather the fact that the spun yarn rotates along at approximately the rotational speed of the rotor in the manner of a crank and in the process slides essentially on the contact surface of the yarn withdrawal nozzle. The contact point of the yarn on the contact surface therefore changes constantly so that an effective dissipation of frictional heat on it is always possible.
This solution according to the invention differs clearly from the known solution (German Patent Document DE 24 10 940 C3), specifically of cooling by means of an air flow, the yarn guiding funnel, which can be compared with a yarn withdrawal nozzle. Such an air cooling cannot prevent the local overheating of the fibers because of the generated frictional heat because it is not effective at the point at which the frictional heat is generated, specifically between the contact surface and the corresponding fibers. The cooling of such an air flow therefore always comes too late because it only becomes effective after the frictional heat has already caused the increased temperature in the fibers. The resulting damage can no longer be reversed.
Tests carried out by and in behalf of applicants have shown that, for example, by means of yarn withdrawal nozzles made of copper which has a very high thermal conductivity, it is possible to increase the rotational rotor speeds and thus the spinning speed without any damage to the fibers by at least 20% compared to the previously possible rotational rotor speeds.
In a further development of the invention, a basic body is provided for the yarn withdrawal nozzle which is coated with the material at least in the area of the contact surface. Since in every case only a short-term but fast heat dissipation is required, it is to be expected that also a relatively thin coating of less than 0.5 mm will be sufficient for obtaining the desired effect.
In a further development of the invention, it is provided that the contact surface has a hardness of at least 20 HV.sub.5N. Thus it is ensured that a sufficiently high resistance to wear exists so that such a yarn withdrawal nozzle also has a sufficient useful life.
In a further development of the invention, it is provided that the material is titanium diboride. This material has the two characteristics which are the most desirable, specifically a high heat conductivity in a temperature range of approximately 50.degree. to approximately 100.degree. C., while it also has a high hardness.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.